expose transform_to_head in API docs

Author: sappelhoff

doc/api/preprocessing.rst

@@ -48,6 +48,7 @@ Projections:
    read_dig_localite
    make_standard_montage
    read_custom_montage
+   transform_to_head
    compute_dev_head_t
    read_layout
    find_layout

mne/channels/__init__.pyi

@@ -32,6 +32,7 @@ __all__ = [
     "read_polhemus_fastscan",
     "read_vectorview_selection",
     "rename_channels",
+    "transform_to_head",
     "unify_bad_channels",
 ]
 from .channels import (
@@ -73,4 +74,5 @@ from .montage import (
     read_dig_localite,
     read_dig_polhemus_isotrak,
     read_polhemus_fastscan,
+    transform_to_head,
 )